I realize what follows is actually an argument for restricting stride to collections with randomly accessible elements, and maybe we should:
We've touched a little bit on performance, and I think my feeling with stride is that just the name itself suggests a certain logic--namely, that we actually skip over, rather than visit and discard, the elements that aren't in the sequence. I form this intuition from the ordinary sense of the word "stride"--if my walking gait has a stride size of two feet and there's a puddle less than one foot wide right in front of me, striding by two feet means that my feet stay dry. It doesn't mean I drag one shoe through the puddle and ignore it. Likewise, when I stride from 2 to 10 by 2, I'm adding two at every step, not adding one twice. Since an ordinary user of stride doesn't and shouldn't have to inspect the code in the stride iterator, I think it would violate some users' expectations if sequences that are not collections have each element visited regardless of stride size. A user can trivially write a for loop iterating over the sequence itself and discard every not-nth element. We shouldn't offer a stride function that looks more performant but actually isn't. On Mon, Apr 11, 2016 at 7:38 PM Dave Abrahams <dabrah...@apple.com> wrote: > > on Sun Apr 10 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote: > > > On Sun, Apr 10, 2016 at 3:58 PM, Haravikk <swift-evolut...@haravikk.me> > wrote: > >> > >> On 10 Apr 2016, at 14:25, Xiaodi Wu <xiaodi...@gmail.com> wrote: > >> > >> What types do you have in mind that would only support positive > distances? > >> All numeric types (yes, even UInt, etc.) have signed distances, which > > > >> reflects the basic mathematical abstraction of a number line. > >> > >> > >> Say you wanted to stride through a singly-linked list, it would > actually be > >> beneficial to support only forward strides, the same is true of > sequences, > >> as you either may not know what the endpoint is, or would have to step > >> through the whole sequence to find it (plus buffer every value in order > to > >> do-so safely). > >> > >> A consistent behavior with signed distances is so important that we are > >> currently struggling with an interesting issue with floating point > types, > >> which is that due to rounding error 10.0 + a - a != 10.0 for some > values of > >> a. > >> > >> > >> While that’s interesting I’m not sure why the sign is important; to me a > >> stride is a width so it being negative makes no sense. For example, say > I > >> laid an array of Ints, organised into groups of five (and also that I’m > >> lunatic who won’t use a tuple for this), the stride of this array is 5 > >> whether I’m stepping through it forwards or backwards. Imagine I defined > >> this like so (more realistically it’d be a struct or a class): > >> > >> typealias StridedIntegerArray:(stride:Int, array:[Int]) > >> > >> If the stride is set to 5, it’s always 5, the only thing that changes is > >> whether I want to stride from the start or end of the array, plus I > could > >> things like: > >> > >> myStridedIntegerArray.prefix(from: 2).striding(forwardBy: > >> myStridedIntegerArray.stride) // Returns element at index 2, 7, 12, etc. > > > > When you have a sequence returning elements at index 12, 7, 2, etc., > > wouldn't you call the stride size -5? I would, because 12 + (-5) = 7. > > > >> > >> > >> It just occurred to me that perhaps you intended this method only for > ranges > >> specifically and that perhaps I’m confusing things, but it seems to me > like > >> it should be a method for all sequences (with reverse stride available > on > >> collections with a reverse index type) returning a generator that only > >> returns (or computes) every Nth element, for generic > sequences/collections > >> this would take the start or end index and use advanced(by:), though > again, > >> I kind of feel like that should be two separate methods as well, but > that’s > >> for another issue I think. > > > > I don't think it should be for ranges only, but ranges are the extent > > of this proposal. > > > > That said, my own opinion is that striding should not be available on > > sequences but on collections only. In their most commonly used form, > > integer strides take a start and end, and there is a finite number of > > things to stride over; thus, in my reasoning, strides can be extended > > to cover anything else that has a known start and end and has a finite > > number of things, which is guaranteed by conformance to Collection but > > not to Sequence. > > I dunno; it seems to me that if someone gives me a Sequence I should be > able to traverse it, skipping every other element. I don't see why > “stride” should be inapplicable here. > > > (At the moment, StrideTo/Through conforms to Sequence and not to > > Collection, but that is considered something to be fixed and we will > > see if we can address that as part of this set of stride overhauls.) > > > > As I see it, we agree on the problem: the current algorithm cannot > > accommodate singly linked lists and sequences because those things do > > not have a known endpoint if you begin an attempt to stride. However, > > my conclusion is the opposite of yours: namely, that they should not > > have stride. Maybe they should have something similar, but it > > shouldn't be stride. > > > >> > >> On Sun, Apr 10, 2016 at 12:53 PM Haravikk via swift-evolution > >> <swift-evolution@swift.org> wrote: > >>> > >>> > >>> On 10 Apr 2016, at 11:17, Brent Royal-Gordon <br...@architechies.com> > >>> wrote: > >>> > >>> Why not just assign it the correct sign during the init function? > >>> (0 ... 6).striding(by: 2) // [0, 2, 4, 6], end > start, so stride = by > >>> (6 ... 0).striding(by: 2) // [6, 4, 2, 0], start > end, so stride = -by > >>> > >>> > >>> One reason not to do it this way is that, if we extend `striding(by:)` > to > >>> other collections, they will not be as easy to walk backwards through > as > >>> this. You will have to do something like > >>> `collection.reversed().striding(by:)` which will be a hassle. > >>> > >>> > >>> Any thoughts on the alternative I mentioned a little earlier to define > >>> overloads instead of positive/negative? i.e- you would have two > methods, > >>> .striding(forwardBy:) and .striding(backwardBy:). In addition to > eliminating > >>> the use of a negative stride to indicate direction, this has the > advantage > >>> that .striding(backwardBy:) can be defined only for types with a > >>> ReverseIndex or only for collections (as you can stride through a > sequence, > >>> but only by going forward). > >>> > >>> This should also make documentation a bit clearer, otherwise you’ve got > >>> the caveat that to go backwards requires a negative value, but only if > the > >>> type supports that, which a developer would then need to check. > Instead it > >>> either has the backwardBy variant or not. > >>> > >>> I know that advance(by:) supports negative values, but this is actually > >>> something I wouldn’t mind seeing changed as well, as it has the same > issues > >>> (passing a negative value in looks fine until you realise the type is a > >>> ForwardIndex only). It would also allow us to define Distance types > that > >>> don’t support a direction, since this would be given by the choice of > method > >>> called instead. > >>> > >>> > >>> Of course I’d still like to be able to define 6 … 0 or whatever, but > this > >>> would at least eliminate what I dislike about using negatives for > direction. > >>> _______________________________________________ > >>> swift-evolution mailing list > >>> swift-evolution@swift.org > >>> https://lists.swift.org/mailman/listinfo/swift-evolution > >> > >> > > -- > Dave >
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